fix reduce_vars on AST_Arrow (#2091)

fixes #2090

lib/compress.js

@@ -363,7 +363,7 @@ merge(Compressor.prototype, {
                     safe_ids = save_ids;
                     return true;
                 }
-                if (node instanceof AST_Function) {
+                if (node instanceof AST_Function || node instanceof AST_Arrow) {
                     push();
                     var iife;
                     if (!node.name

test/compress/reduce_vars.js

@@ -2537,3 +2537,53 @@ accessor: {
     }
     expect_stdout: "1 1"
 }
+
+issue_2090_1: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+    }
+    input: {
+        console.log(function() {
+            var x = 1;
+            [].forEach(() => x = 2);
+            return x;
+        }());
+    }
+    expect: {
+        console.log(function() {
+            var x = 1;
+            [].forEach(() => x = 2);
+            return x;
+        }());
+    }
+    expect_stdout: "1"
+    node_version: ">=4"
+}
+
+issue_2090_2: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+    }
+    input: {
+        console.log(function() {
+            var x = 1;
+            [].forEach(() => {
+                x = 2;
+            });
+            return x;
+        }());
+    }
+    expect: {
+        console.log(function() {
+            var x = 1;
+            [].forEach(() => {
+                x = 2;
+            });
+            return x;
+        }());
+    }
+    expect_stdout: "1"
+    node_version: ">=4"
+}